How to make a brush (alpha image) using Cycles
First Render out the image. The lights and materials don't matter.

In the compositor add a normalize node and drop the z pass through that. This will make the lowest value 0, and the highest 1 while mapping the rest through that.

Then with the RGB Curves node, switch the handles so that they are inverted (left being on the top, and right on the bottom), and add a handle in the middle and adjust it until you're happy with the result.

 
The finished node setup should look something like that